Force-induced melting of the DNA double helix - 1. Thermodynamic analysis

The highly cooperative elongation of a single B-DNA molecule to almost twic e its contour length upon application of a stretching force is interpreted as force-induced DNA melting. This interpretation is based on the similarit y between experimental and calculated stretching profiles, when the force-d ependent free energy of melting is obtained directly from the experimental force versus extension curves of double- and single-stranded DNA. The high cooperativity of the overstretching transition is consistent with a melting interpretation. The ability of nicked DNA to withstand forces greater than that at the transition midpoint is explained as a result of the one-dimens ional nature of the melting transition, which leads to alternating zones of melted and unmelted DNA even substantially above the melting midpoint. We discuss the relationship between force-induced melting and the B-to-S trans ition suggested by other authors, The recently measured effect on T7 DNA po lymerase activity of the force applied to a ssDNA template is interpreted i n terms of preferential stabilization of dsDNA by weak forces similar to7 p N,